2.1.4. Porosimetry
Mercury intrusion porosimetry (MIP) and gas intrusion (BET) are used to determine effective porosity, pore distribution, surface porosity, and particle size of small intact samples of mudrocks. The characterization of those microtextural features is important as they are closely related to breakdown processes developed in mudrocks. Specialised equipment is used for the determinations.
2.1.5. Chemical Analyses
Several techniques, including wet chemical, Inductively Coupled Plasma Atomic Absorption Spectrometry (ICP-AAS), Atomic Absorption (AA), or X-ray Fluorescence (XRF) methods, may be used to determine bulk chemical analysis for both major and trace elements present in mudrocks (see BS1377-3:2018,
[10]). Among those, XRF in which a powdered sample of the rock or soil is analysed, is a rapid procedure providing analysis of most mudrock components. However, chemical data have limited value in weathering assessment as only small changes in chemical composition accompany physical weathering processes if these predominate.
The amount of pyrite present in a mudrock sample can be determined by determining the total sulphur content by High-Temperature Combustion and then treating a sample with acid to remove any acid-soluble sulphur and analysing the resulting solution using ICP atomic emission spectroscopy or other means. In most geological materials, gypsum is the acid-soluble sulphur compound and pyrite is the only acid-insoluble sulphur compound, so the amount of pyrite can be calculated from the difference between total sulphur and acid-soluble sulphur.
2.2. Identification Test
2.2.1. Density and Porosity
Density and total and effective porosity may be determined by several testing procedures provided by the International Society for Rock Mechanics (ISRM)
[11]. Alternative methods, such as mercury porosimetry, are used on samples that disintegrate under vacuum saturation.
2.2.2. Natural Water Content
The natural water content gives valuable information concerning the presence of hydrophilic compounds in rock, particularly clay minerals. Thus clay-rich mudrocks, especially those containing swelling species, have relatively high natural water contents. Water absorption and water adsorption contents are, respectively, determined by conducting immersion tests and exposure of samples of rock fragments to specific moisture conditions
[11].
2.2.3. Particle Size and Atterberg Limits
Prior disaggregation of the material is necessary for the determination of particle size distribution
[12] and Atterberg limits
[13]. Methods of disaggregation include alternate wetting and drying, the use of acids or chemical dispersing agents, and mechanical crushing. However, inter-particle bonding in mudrocks may prevent the disaggregation of the material into individual particles, in which case the values obtained in these tests are highly dependent on the effectiveness of the disaggregation procedure.
2.2.4. Methylene Blue Adsorption
The methylene blue adsorption spot test may be used to evaluate the hydrophilic surface characteristics of the clay minerals and, thus, their capacity to retain water. Methylene blue is not adsorbed by inert minerals and thus may be used as a routine test to assess the swelling clay component in a powdered rock sample
[14].
2.3. Strength and Deformability
2.3.1. Uniaxial Compressive Strength Tests
Uniaxial compressive strength procedures are given by ISRM
[15] and ASTM
[16]. The preparation of cylindrical or prismatic specimens for the test comprises the major drawback of this test in mudrock, especially material containing laminations that hamper the preparation of specimens with dimensions suitable for the test. The tests also entail drying the specimens, which may cause them to become damaged.
2.3.2. Tensile Strength Tests
Testing procedures for tensile strength are given in ISRM
[17], but preparing samples and applying tensile forces to specimens are very challenging, so often diametral compression (Brazilian) tests are carried out. However, particularly on massive mudrocks, as diametral testing pre-determines the specimen failure surface, they give higher values than direct tensile strength tests.
2.3.3. Point Load Test
The point load strength test procedure is described in ISRM
[18]. Although the damage to specimens is caused by coring or cutting equipment, the test is most suitable for testing indurated strong mudrock types. It is not appropriate for weak or weathered materials. Poor results are obtained for irregular block mudrock samples with an axial distance smaller than 25 mm. Equidimensional lumps tested with loading direction perpendicular to stratification or weakness planes provide the most consistent results. However, for mudrock strength, anisotropy assessments with loading directions normal and parallel to bedding must be carried out.
2.3.4. Schmidt Rebound Test-Hammer
The testing procedures for the Schmidt rebound test hammer are given by ISRM
[19]. This equipment is mostly used for field testing of rock outcrops, but it is also used for laboratory testing on core and/or block samples. Determinations of rebound number using an N19 Schmidt rebound test hammer for UK mudstones are provided by Carter and Sneddon
[20].
2.3.5. National Coal Board Cone Indenter (NCB)
The National Coal Board (NCB) cone intender
[21] was developed to determine the strength of rock chips not greater than 12 × 12 × 6 mm in size, where the testing procedures are provided by the National Coal Board, UK
[21]. It is a portable device that does not require elaborate specimen preparation. It is very suitable for testing thinly bedded or fractured mudrock fragments that would break up during the preparation of specimens for uniaxial compression tests. In the test, a steel cone is driven against the rock fragment until the deflection values of the steel beam reach 0.635 mm (standard test) or 0.23 mm (soft rock test).
2.4. Swelling
2.4.1. Swelling Strain
Axial swelling strain tests on radially confined remoulded specimens performed following the National Laboratory for Civil Engineering (LNEC) test standard E- 200
[22] provide data about the swelling of the mineralogical constituents of the material in the presence of water. The apparatus is illustrated in
Figure 4a, where the sample consists of two 15 mm thick compacted layers of dry disaggregated material passing a #40 ASTM (425 μm) sieve. The compaction is performed using a specific plunger applying a force of 0.5 MPa. A micrometer dial gauge reading of 0.01 mm is used to record swelling strain resulting from immersion in water over a test time of 48 h.
Figure 4. Diagrammatic illustrations of the apparatus used to measure swelling strain
[23]: (
a) on radially confined remoulded specimens; (
b) on rock specimens.
Uniaxial and triaxial swelling strain test procedures for intact rock specimens using the apparatus shown in
Figure 4b are given by ISRM
[11]. One of the orthogonal axes is perpendicular to the bedding or parting in the rock. The test specimens consist of pre-cut cubes of approximately 30 mm side lengths, and they are mounted with the z-axis normal to the bedding. Micrometer dial gauges reading to 0.001 mm record swelling strain due to immersion in water during a standard test time of 48 h or until swelling has ceased. Problems arise with cutting the cubes in weak and low-durability materials.
2.4.2. Swelling Stress
Swelling stress–strain test procedures are provided by ISRM
[24], which require that the applied force is increased during the test to prevent the specimen from increasing in volume during the test. The apparatus described by Jeremias
[23] consists of a rigid frame and an electrical load cell, as shown in
Figure 5. The swelling stress developed under conditions of zero volume change on Portuguese mudrock samples of Cretaceous age is shown in
Figure 6 [25].
Figure 5. Apparatus for measuring the axial swelling stress of an undisturbed radially confined rock specimen
[23].
Figure 6. Axial swelling pressure versus time plots of Cretaceous Portuguese mudrocks measured perpendicular (black) and parallel (red) to bedding
[25]. σ∗ —Maximum axial swelling stress.
2.5. Durability
2.5.1. Ageing Tests
Natural exposure and ageing tests provide means of mudrock durability assessment. Details of the long-term disintegration behaviour of mudrocks subjected to natural exposure tests are given by Shakoor and Gautam
[26]. In ageing tests, a specific weathering process is reproduced, of which the most common are cyclic wetting and drying, freezing and thawing, and soundness tests. Although these tests are used in research studies, they are very time-consuming and not commonly used in routine investigations.
2.5.2. Slake Durability Test
Mudrock durability assessment in routine studies is usually based on slaking due to cyclic wetting and drying and mechanical disturbance evaluated using the slake durability test apparatus proposed by Franklin and Chandra
[27]. This test was further recommended by ISRM
[11] and standardized by ASTM
[28]. The two-cycle slaking durability index (I
d2) has been adopted in several classification schemes for mudrock durability assessment. For the test, 10 equidimensional dried rock lumps, each 40–60 g mass, are placed in a cylindrical drum formed out of 2 mm mesh that is rotated on a horizontal axis while partly submerged in a water bath. The amount of sample retained in the drum after 200 rotations is collected, dried, and expressed as a percentage of the original dry mass of the sample.
2.5.3. Static Slake Test
The static slake test consists of one cycle in which an oven-dried 40–50 mm sized cube of rock in which one face is perpendicular to the bedding, is immersed in water, and its behaviour is observed at specific times over a period of 24 h.
Several classification schemes have been proposed
[29][30][31][32] to categorise the slaking behaviour of mudrocks. Santi
[33] linked static slake categories to the types of slaking observed and proposed a six-category classification providing a standard visual basis for distinguishing between degrees of chip or fracture formation. Accordingly, category 1 refers to degradation to a mud-like consistency, and categories 2 and 3, respectively, describe the formation of flakes and chips. Categories 4 and 5 describe the formation of fractures and slabs, and category 6 is applied when no reaction is observed.
2.6. Compaction Tests
In this context, compaction tests are used to study the change of grading between uncompacted and compacted material, which reflects the breakdown of the particles during the compaction process. Several procedures have been proposed for this mudrock degradability assessment, but the test given by NF P94-066
[34] is one of the most used. In this test, degradability is expressed as the ratio between D
10 determined in the initial grading curve for the sample and of D
10 obtained in the grading curve after sample compaction with one hundred blows with a standard Proctor hammer using a CBR mould. Ratios higher than 7 express materials are prone to breakdown.
3. Mudrock Classifications
3.1. Geological Classifications
The descriptive schemes used for mudrock geological classification are based on features with some genetic significance. Table 1 shows the guidance given by Czerewko and Cripps [35] for the description of mudrock key features. Colour, mineralogy, fossil content, fracture type, and induration state are descriptive modifiers that complement the root names for a better mudrock characterization.
Table 1. Guide to the description of mudrock features
[35].
Attribute |
Descriptive Adjectives |
Induration |
Enables decision on description as soil or rock. If resistant to slaking in water and hard, it is rock; if susceptible to slaking in water, deformable, and ‘earthy consistency, it is soil. Strength depends on moisture state; dry sediment is stronger than wet, and rock strength varies with moisture content; sampling may impair strength. |
Strength |
Strength is designated based on the degree of induration. For soil, use field consistency values based on manual assessment, e.g., stiff; when shear strength measurements are made, use strength terms, e.g., high strength. For rock, a definition based principally on manual field assessment using geological hammer and knife may be confirmed with UCS measurement: indurated mudrocks range from extremely weak to medium strong; metamudrocks are stronger depending on weathering. |
Structure |
Standard terms for beds, laminae, and parting are provided by Potter et al. [1]. Include description of lithology and textural inter-relationship, as complex features may be present with structured strata such as ‘thin beds of cross bedded’ mudstone. |
Colour |
Use Munsell colour chart for consistency. Important for correlation; likely environment of formation and indication of likely behaviour of material, i.e., red colour—likely formation under oxidizing continental environment. Most important to mudrocks is relationship between colour on the Fe3+/Fe2+ ratio. A decrease in this ratio gives an increase in colour from red → green → grey (more Fe2+ indicates the presence of pyrite). Organic carbon controls colour: <0.2–0.3%C = light-grey to olive grey; 0.3–0.5%C = mid-grey; >0.5%C = dark-grey to black. |
Accessory minerals |
Calcareous (slightly to very, based on level of effervescence when assessed with HCl, carbonaceous, dolomitic, ferruginous, glauconitic, gypsiferous, pyritic, micaceous, sideritic, phosphatic. |
Rock name |
See classification of Figure 1. |
Additional information |
Presence of fossils—record type (generic such as bivalve and retain for identification), abundance, condition, orientation. Inclusions—nodules (with mineral type and details); gravel, sand, silt partings or pockets, etc. |
State of weathering |
Alteration seen as distinct discoloration, significant strength reduction to discontinuities, and presence of lithorelicts (note orientation). |
Fractures |
Use ISO 14689:2017 standard terms and procedures [36]. For rock supplements with details such as nature of fragmentation, e.g., conchoidal, hackly, brittle, splintery, slabby, fissile. |